1. Field of the Invention
The present invention relates to an apparatus and process for forming a diamond film and, more particularly, to an apparatus and process for synthesizing a diamond film out of a vapor phase by making use of an arc discharge.
2. Description of the Prior Art
In recent years, various processes have been proposed as methods of synthesizing diamond films out of a vapor phase at low pressures. One process is the hot filament CVD. In particular, a tungsten filament is placed just above a substrate which is heated to 800.degree. to 1000.degree. C. The filament is heated above 2000.degree. C. Hydrogen and a hydrocarbon gas (e.g., CH.sub.4) are blown against the substrate through the filament to grow a diamond film on the substrate.
Another known process is the microwave plasma CVD. In this process, a plasma is produced within a mixture gas of hydrogen gas and hydrocarbon gas by application of microwaves of several hundred watts. A diamond is grown on the substrate placed within the plasma. The substrate is heated to about 700.degree. to 900.degree. C. by the microwaves.
In these two processes for synthesizing diamond, hydrogen atoms act to promote decomposition of CH.sub.4 and to selectively etch synthesized materials other than the diamond such as amorphous carbon. In this way, hydrogen atoms play an important role. However, the hot filament CVD is not practical, because the heated filament often breaks. When the melting point of tungsten is taken into account, the filament cannot be heated above about 2000.degree. C. If the filament is heated above this temperature, the filament wire will break. As a result, the raw material gases cannot be decomposed sufficiently. In the process utilizing a microwave plasma, limitations are imposed on the dimensions of the plasma chamber. Therefore, it is difficult to apply this process to a sample having a large area. In addition, the raw material gases, especially hydrogen, are not decomposed sufficiently.
A further process available uses an ion beam. In this process, a beam of carbon ions is caused to bombard a substrate to grow a diamond film. Unfortunately, the produced diamond contains a large amount of impurities such as amorphous carbon.
A process utilizing an arc discharge has been proposed in U.S. Pat. No. 4,851,254. Specifically, an arc discharge is induced between opposite electrodes. The raw material gases are passed through the arc discharge to create a gas plasma. This gas plasma is narrowed down by a restrictor portion and blown as a plasma jet gas against a substrate. Thus, diamond is deposited on the substrate.
This method of synthesizing diamond by making use of an arc discharge can indeed provide a greatly improved synthesis rate, but the ratio of the hydrocarbon introduced for improving the purity of the diamond film to the hydrogen must be decreased. This lowers the synthesis rate. More specifically, as can be seen from the graph of FIG. 18, as the ratio of the hydrocarbon to hydrogen is decreased, the thermal diffusivity of the synthesized diamond film, or the purity, is improved but the synthesis rate drops.
Another problem with this process using an arc discharge is that the gases are not sufficiently dissociated near the substrate against which the raw material gases are blown. Specifically, the use of the arc discharge greatly improves the gas decomposition rate, but this improvement is accomplished only around the generated arc discharge. The gas decomposition rate is much lower around the substrate against which the plasma jet gas is blown because of recombination and other reactions. In FIG. 19, the spectral intensity of the emission spectrum H.beta. of hydrogen Balmer series representing the amount of the hydrogen radicals, or the decomposed hydrogen, is plotted against the distance from the nozzle, or the plasma ejection port, to the substrate. As can be understood from this graph, the gas decomposition rate drops rapidly away from the plasma ejection port. That is, the decomposition rate is much lower around the substrate.
Bringing the substrate close to the plasma ejection port to synthesize the diamond at positions of high gas decomposition rates may be contemplated. However, this elevates the substrate temperature, rapidly increasing the amount of impurity carbons other than diamond such as graphite. This is because the appropriate temperature of the substrate for the synthesis of a diamond is between 600.degree. C. and 1100.degree. C. and because the optimum temperature for high-purity synthesis is between 800.degree. C. and 1000.degree. C. Therefore, in order to synthesize a diamond at a higher efficiency, it is necessary to enhance the gas decomposition rate around the substrate without elevating the substrate temperature.